Composite filler particles and process for the preparation thereof

ABSTRACT

A process for the preparation of composite filler particles, comprising: (a) coating a particulate filler having a median particle size (D50) of from 1 to 1200 nm; (b) agglomerating the coated particulate filler, for providing a granulation of the coated particulate filler wherein the granulation contains the coated particulate filler particles whereby the at least one coating layer may be crosslinked by crosslinking groups obtained by reacting the reactive groups and optionally a further crosslinking agent.

RELATED APPLICATIONS

This patent application is a divisional patent application of U.S.patent application Ser. No. 13/714,803, now U.S. Pat. No. 9,193,849,which claims the benefit of and priority to EP Application Ser No.11009866.2, filed on Dec. 15, 2011, which is herein incorporated byreference for all purposes.

FIELD OF THE INVENTION

The present invention relates to a process for the preparation ofcomposite filler particles. Moreover, the present invention also relatesto composite filler particles obtainable by the process according to thepresent invention. Furthermore, the present invention relates to the useof the composite filler particles in a dental composition such as adental restorative material. The composite filler particles of theinvention have a high content of inorganic filler for providing improvedmechanical properties and reduced polymerization shrinkage withoutimpairing workability due to an increase of viscosity. Moreover, thecomposite filler particles provide excellent aesthetic properties due toimproved surface properties of a dental restoration prepared by aprocedure including a polishing step. The composite filler particleshave an essentially spherical shape.

BACKGROUND OF THE INVENTION

Dental restorative materials for use in the preparation of crowns,veneers, direct fillings, inlays, onlays and splints are known. Dentalrestorative materials such as dental composites contain a curable resinand particulate filler. However, resin shrinkage upon polymerization inthe curing process tends to cause gap formation between the restorativecomposite and the tooth. As a consequence micro leakage, secondarycaries and decreased longevity of the repair represents a problem withprior dental restorative materials. In order to alleviate the shrinkingproblem and to reinforce dental restorative materials, particulatefillers are commonly used, whereby a high filler load is preferred.

EP 1 396 254 discloses a particulate prepolymerized filler prepared bymixing an inorganic filler with an organic polymerizable resin andcuring the mixture. Specifically, fumed silica and a bariumaluminoborosilicate glass are mixed with polymerizable resin to preparea paste and the paste is then heat polymerized and the resultantpolymerized mass is ground to the desired particle size, for example,using a ceramic ball mill. The prepolymerized filler disclosed by EP 1396 254 has an inorganic load of about 55 percent by volume and is usedto enable higher filler loading of a dental restorative compositionwhile maintaining acceptable handling properties of the paste.

However, the prepolymerized filler of EP 1 396 254 is problematicregarding the mechanical and chemical resistance compared to theproperties of the particulate glass which is used for preparing theprepolymerized filler.

Moreover, since the prepolymerized filler must be ground and classifiedbefore incorporation into a dental restorative composition, theprepolymerized filler of EP 1 396 254 requires additional time andenergy consuming process steps.

WO 0130304 discloses filler particles for use in a dental restorativecomposition, which comprise clusters of nano-sized metal oxide particlesand further non-agglomerated nano-sized particles. The clusters are madeusing a process that includes heat treatment of a spray dried sol ofmetal oxide particles. The filler particles are silanated andincorporated into a dental restorative composition in an amount of about78 parts by weight. WO 0130304 suggests that the clusters providestrength, while the nano-sized particles provide aesthetic quality,polishability, and wear resistance. However, the filler is problematicin that spray-drying and calcining metal oxide sol particles provide aproduct which requires milling in a ball mill for 160 hours in order toachieve an average cluster size of 1 μm.

The particulate filler materials of the prior art are not satisfactorywhen used in a dental restorative material having high filler loadingfor reduced shrinkage while being mechanically strong. A high fillerloading results in a viscosity problem, which may usually be addressedby using large filler particles. Large filler particles generallyprovide a lower viscosity as compared to smaller filler particles.However, a cured product of a composition containing large fillerparticles is unsatisfactory regarding the polishability since a smoothsurface may hardly be obtained due to large filler particles beingremoved from the surface of the cured product leaving cavities impairingthe aesthetic properties.

U.S. Pat. No. 6,020,395 discloses a homogeneous microfilled dentalcomposite material comprised of a mixture of polymerizable monomers andan inorganic filler, wherein said filler is comprised of silane treatedfused silica aggregates having a size ranging from submicron to about160 μm. The aggregates are comprised of agglomerates of fumed silicahaving an average agglomerate size in the range of approximately 0.5 to50 μm, and the agglomerates are comprised of primary particles of fumedsilica having an average particle size in the range of approximately 1to 100 nm. The primary particles are interconnected by siloxane bridgesformed by burning an organosilane coating on the fumed silica. Accordingto Example 1, raw OX-50 was coated with 20% by weight A-174 organosilanein a V-blender using an aqueous solution spray, dried in a forced airoven at 100° C. for 24 hours, and hammermilled to a 10 μm averageparticle size. The silane-coated OX-50 was oxidized at 1050° C. for 4hours, resulting in bridged silica particles (fused silica). The fusedsilica was then surface treated with 7% by weight A-174 organosilane ina V-blender using an aqueous solution spray. The silane-treated fusedsilica was dried at 110° C. for 3 hours and at 55° C. for 16 hours, thensieved through 95 mesh. The resulting filler consisted of agglomeratesof Si—O bridged 0.04 μm fumed silica with aggregates of 10 μm mean sizeand a range of from submicron to 160 μm.

U.S. Pat. No. 4,781,940 discloses a process for the production of afiller for use in a microfilled dental composite formulation, whichprocess comprises the steps of: (a) coating colloidal silica with apolymerizable monomer by mixing said silica with an organic solventsolution of said monomer and an effective amount of a polymerizationcatalyst, and then evaporating said solvent; (b) individualizing thecoated silica by screening to product particles having a maximum size ofabout 90 microns; (c) polymerizing said monomer; and (d) individualizingthe coated silica particles comprising the product of step (c) byscreening. The filler of U.S. Pat. No. 4,781,940 consists of particleshaving an irregular shape thereby causing undesireably high viscosityincreases when used in a dental composition.

SUMMARY OF THE INVENTION

It is a problem of the present invention to provide composite fillerparticles which affords high filler loading, high strength and excellentpolishability after curing of a dental restorative material containingthe particulate composite filler while maintaining appropriate viscosityfor good workability of the dental restorative material, and lowershrinkage during polymerization, as well as process for the preparationof a particulate filler.

The present invention provides a process for the preparation ofcomposite filler particles, comprising:

(a) coating a particulate filler having a median particle size (D50) offrom 1 to 1200 nm with a coating composition containing a film-formingagent forming a coating layer on the surface of the particulate filler,said coating layer displaying reactive groups on the surface of thecoating layer, said reactive groups being selected from additionpolymerizable groups and step-growth polymerizable groups, therebyforming a coated particulate filler; subsequently or concurrently(b) agglomerating the coated particulate filler, optionally in thepresence of a further crosslinking agent and optionally in the presenceof a further particulate filler not displaying reactive groups, forproviding a granulation of the coated particulate filler wherein thegranulation contains the coated particulate filler particles and theoptional further particulate filler particles separated from andconnected to each other by at least one coating layer, whereby the atleast one coating layer may be crosslinked by crosslinking groupsobtained by reacting the reactive groups and optionally a furthercrosslinking agent;(c) optionally milling, classifying and/or sieving the granulation ofthe coated particulate filler; and(d) optionally further crosslinking the granulation of the coatedparticulate filler; for providing composite filler particles having amedian particle size (D50) of from 1 to 70 wherein reactive groups aretransformed into crosslinking groups obtained by reacting reactivegroups and optionally a further crosslinking agent, and wherein theparticulate filler is the main component by volume of the compositefiller particles.

According to the present invention, agglomeration may be carried out byspray agglomeration or growth agglomeration, whereby spray agglomerationis preferred.

According to the present invention, the reactive groups of thegranulation of the coated particulate filler comprised in the compositeparticulate filler is partially crosslinked. Crosslinking may be due tocrosslinking groups obtained by reacting the reactive groups andoptionally a further crosslinking agent in the step of agglomerating thecoated particulate filler, optionally in the presence of a furthercrosslinking agent and optionally in the presence of a furtherparticulate filler not displaying reactive groups, for providing agranulation of the coated particulate filler. Alternatively oradditionally, crosslinking may be due to crosslinking the granulation ofthe coated particulate filler after the agglomeration step (a), or afterthe optional step of milling, classifying and/or sieving the granulationof the coated particulate filler.

Moreover, the present invention also provides a particulate compositefiller obtainable by the process according to the invention. Theparticulate composite filler of the present invention comprisesgenerally spherical primary composite filler particles, in particulargenerally sherical primary composite particles oibtainable by sprayagglomeration. Primary particles are particles which cannot be reducedin size by breaking up aggregated particles, for example, by usingsonication.

The particulate composite filler of the present invention comprisesresidual reactive groups such as polymerizable double bonds due to theincomplete crosslinking of reactive groups. The residual reactive groupsmay react with further reactive groups of a curable matrix in which theparticulate composite filler is dispersed. Specifically, residualpolymerizable double bonds of the particulate composite filler of thepresent invention may react with a polymerizable matrix of a dentalcomposition when the particulate composite filler is used in a dentalcomposition.

Furthermore, the present invention provides a use of the particulatecomposite filler of the present invention in a dental composition.

The present invention is based on the recognition that it is possible toefficiently and effectively prepare a composite filler particles havinga high inorganic filler load by coating primary particles of aparticulate filler with a coating composition containing a specificreactive film-forming agent forming a coating layer on the surface ofthe particulate filler. Based on the coated particulate filler, agranulation may be provided wherein the granulation contains the coatedparticulate filler particles separated from each other by at least onecoating layer. Crosslinking of the reactive groups of the specificreactive film-forming agent stabilizes the granulation by covalentbonding whereby composite filler particles of the present invention areobtained.

According to the process of the present invention, it is not necessaryto use time and energy consuming milling steps while at the same time,the process of the present invention provides control over the particlesize distribution so that a large amount of fines or coarse particles asside products are avoided.

The composite filler particles may be obtained with a large particlesize for incorporation into a highly filled dental composition wherebythe viscosity and the workability of the dental composition areexcellent. A cured dental composition provides a material having a highfiller loading, high strength and excellent polishability and shows lowshrinkage during polymerization.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a flow chart illustrating a first generic embodiment of theprocess of the present invention based on a suspension/emulsiontechnique.

FIG. 2 is a flow chart illustrating a second generic embodiment of theprocess of the present invention wherein a spray dryer is used.

FIG. 3 is a flow chart illustrating a third generic embodiment of theprocess of the present invention based on wherein a high shear mixer ora fluidized bed is used.

FIG. 4 shows SEM images of agglomerated fillers according to the presentinvention.

FIG. 5 shows an SEM image of a filler according to example 1 of U.S.Pat. No. 4,781,940 using OX-50 as a particulate filler.

FIG. 6 show an SEM image of an agglomerated filler based on OX-50according to the present invention

FIG. 7 shows SEM images of a filler prepared according to the method ofU.S. Pat. No. 4,781,940 except that silanated dental glass as aparticulate filler is used.

FIG. 8 shows SEM images of agglomerated fillers based on a silanateddental glass according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides a composite filler particles. Thecomposite filler particles are particularly useful for the preparationof a dental composition. A dental composition is preferably a dentalrestorative material. The dental restorative material may be selectedfrom a dental composite, a dental cement or a resin reinforced dentalcement. A dental composite may be a highly filled dental composite, aflowable composite, a compomer, a root canal sealer, or a pit andfissure sealant. A dental cement may be a glass ionomer cement or aluting cement.

The particulate filler is the main component by volume of the compositefiller particles. Accordingly, more than 50 percent by volume,preferably more than 60 percent by volume, still more preferably morethan 70 percent by volume of the composite filler particles are occupiedby a particulate filler.

In order to provide a high volume of the particulate filler in thecomposite filler particles, the maximum thickness of the coating layeron the particulate filler is preferably less than the median particlediameter (D50) of the particulate filler. According to a preferredembodiment, the maximum thickness of the coating layer is less than 100percent of the median particle diameter (D50) of the particulate filler,more preferably less than 50 percent of the median particle diameter.More preferably, the maximum thickness of the coating layer is less than20 percent of the median particle diameter (D50) of the particulatefiller.

Preferably, the composite filler particles have a porosity of at most 20percent, preferably at most 15 percent, a still more preferred at most10 percent, as measured by the mercury method in accordance with DIN 66133.

The composite filler particles of the present invention have a medianparticle size (D50) of from 1 to 70 μm, preferably, 2 to 50 μm. Themedian particle size (D50) is measured after any aggregates of thecomposite filler particles have been broken up and dispersed, forexample, by sonication for about 10 minutes in a suitable dispersionmedium. In case agglomeration is carried out by using sprayagglomeration, the median median particle size (D50) is preferably inthe range of from 1 to 40 μm, more preferably, in the range of from 2 to30 μm. In case agglomeration is carried out by using growthagglomeration, the median median particle size (D50) is preferably inthe range of from 30 to 70 μm, more preferably, in the range of from 40to 60 μm.

The particulate composite filler obtained by the process of the presentinvention comprises generally spherical primary composite fillerparticles. The primary particles need not be truly spherical, but shouldat least be rounded to the extent that a fluid-like movement of theparticles is not substantially impeded. The spherical shape of thegenerally spherical primary particles is the result of the agglomerationof the coated particulate filler according to the present invention anddoes not require an additional milling step.

In the composite filler particles according to the present invention,reactive groups are transformed into crosslinking groups by reactingreactive groups and optionally a further crosslinking agent.Accordingly, the initial amount of reactive groups displayed by thecoating layers of the coated particulate filler is reduced in thecomposite filler particles of the invention. The reaction products ofthe reactive groups and optionally the further crosslinking agent may beobserved in the composite filler particles of the present invention.

A residual amount of reactive groups displayed by the composite fillerparticles may be beneficial for the incorporation and crosslinking ofthe composite filler particles in a dental composition. In case, theresidual reactive groups are addition polymerizable groups, e.g.(meth)acryl groups, such residual reactive groups may take part in thecuring reaction based on a radical polymerization of a polymerizableresin matrix of a dental composition in which the composite fillerparticles of the present invention are used. In case, the residualreactive groups are step-growth polymerizable groups, e.g. amino groups,carboxylic acid anhydride groups, or epoxide groups, such residualreactive groups may take part in the curing reaction of a dentalcomposition based on a step-growth polymerization of a step-growthpolymerizable resin matrix containing corresponding step-growthpolymerigable groups.

The residual amount of reactive groups may be determined based on thedegree of polymerization of monomers used for the preparation of thecomposite particulate filler. Preferably the degree of polymerization ofthe monomers is at least 50% and less than 100% and more preferably from80% to 95% as determined by any of the methods disclosed in Example 8.

It is also within the concept of the present invention to treat thecomposite filler particles obtained by the process according to thepresent invention with a silanating agent or surface active agent inorder to modify the surface properties of the composite filler particlesand/or to introduce additional reactive groups or different reactivegroups. Additional reactive groups may be reactive groups capable ofundergoing the same type of polymerization selected from additionpolymerization and step-growth polymerization as the reactive groupsalready displayed by the composite particulate filler. Differentreactive groups may be reactive groups capable of undergoing the othertype of polymerization selected from addition polymerization andstep-growth polymerization as the reactive groups already displayed bythe composite particulate filler.

Suitable silanating agents are any silanating agents which areconventionally used in the dental field, in particular those which aredescribed herein for the preparation of a modified particulate filler,vide infra.

Suitable surface active agents may be selected from surfactants whichpreferable contain one or more reactive groups.

In the composite filler particles, the particulate filler is the maincomponent by volume.

According to the present invention, the reactive groups of thegranulation of the coated particulate filler comprised in the compositeparticulate filler are partially crosslinked. Preferably, crosslinkingis due to crosslinking the granulation of the coated particulate fillerafter the agglomeration step (a), or after the optional step of milling,classifying and/or sieving the granulation of the coated particulatefiller. However, some crosslinking during the agglomeration step (a) maybe occur depending on the conditions used for the agglomeration and thepresence of an initiator system.

The Step of Coating a Particulate Filler

The composite filler particles are prepared according to the presentinvention by a process comprising a step of coating a particulate fillerwith a coating composition.

The coating step may be carried out by dispersing the particulate fillerin a suitable dispersing fluid. The dispersing fluid may be a liquid orgaseous dispersing fluid. If necessary, the particulate filler may bedispersed by using high shear forces. High shear forces may be appliedby mechanical stirring, ultrasonication or atomization using a nozzle.It is preferred that the particulate filler is highly dispersed forincreasing the surface of the particulate filler which is accessible forthe film forming agent during the coating step.

In case the dispersing fluid is gaseous, as examples of the dispersingfluid any gas such as nitrogen, argon, or air may be mentioned as longas the gas does not interfere with the preparation of the compositefiller particles according to the present invention. The gaseousdispersing fluids may used alone or as a mixture of two or more gases.

In case the dispersing fluid is a liquid, as examples of suitableliquids any conventional solvents may be mentioned as long as the liquiddoes not interfere with the preparation of the composite fillerparticles of the present invention, such as tetrahydrofurane,1,4-dioxane, acetone, ethanol, propanol, pentane, hexane, heptane,cyclohexane, toluene, xylene, chloroform, methylene chloride, methylethyl ketone, methyl isobutyl ketone, diethyl ether, tert.-butylmethyland ether. The liquid dispersing fluids may used alone or as a mixtureof two or more liquids in amounts which are miscible with each other.

The dispersion may generally be carried out at a temperature of from −20to 250° C. In case of a liquid dispersing agent, the dispersion may becarried out at a temperature to below the boiling point of the liquidsolvent. Preferably, the dispersion step is carried out at a temperaturein the range of from 10° C. to 150° C. The dispersion may be carried outfor up to 10 hours, preferably from 10 seconds to 1 hour.

The Particulate Filler

The particulate filler has a median particle size (D50) of 1 to 1200 nm,preferably of from 10 to 1000 still more preferably of from 20 to 800 nmas measured using, for example, electron microscopy or by using aconventional laser diffraction particle sizing method as embodied by aMALVERN Mastersizer S or MALVERN Mastersizer 2000 apparatus.

According to a specific embodiment, the particulate filler has a medianparticle size (D50) of 100 to 800 nm.

The particulate filler is not particularly limited as long as thematerial of the particulate filler is acceptable for dentalapplications. Preferable particulate fillers for use in a dentalcomposite may be selected from inorganic particulate filler includingdental glasses, fused silica, quartz, crystalline silica, amorphoussilica, soda glass beads, glass rods, ceramic oxides, particulatesilicate glass, radiopaque glasses (barium and strontium glasses), andsynthetic minerals. It is also possible to employ finely dividedmaterials and powdered hydroxyl-apatite, although materials that reactwith silane coupling agents are preferred. Also available as a fillerare colloidal or submicron oxides or mixed oxides. Suitable inorganicfillers are also YF₃, La₂O₃, ZrO₂, BiPO₄, CaWO₄, BaWO₄ SrF₂, Bi₂O₃.

Preferable particulate fillers for use in a dental cement or a resinreinforced dental cement are reactive. A reactive particulate filler isa powdered metal oxide or hydroxide, mineral silicate, or ion leachableglass or ceramic, that is capable of reacting with an acid in thepresence of water. Examples of particulate reactive filler materialsinclude materials commonly known in the art of glass-ionomer cementssuch as calcium or strontium-containing and aluminum-containingmaterials. Preferably, particulate reactive fillers contain leachablefluoride ions. Specific examples of particulate reactive fillers areselected from calcium alumino silicate glass, calcium aluminofluorosilicate glass, calcium aluminumfluoroborosilicate glass,strontium aluminosilicate glass, strontium aluminofluorosilicate glass,strontium aluminofluoroborosilicate glass. The glass may furthermorecontain zirconium and/or barium. Suitable particulate reactive fillersfurther include metal oxides such as zinc oxide and magnesium oxide, andion-leachable glasses, e.g., as described in U.S. Pat. No. 3,655,605,U.S. Pat. No. 3,814,717, U.S. Pat. No. 4,143,018, U.S. Pat. No.4,209,434, U.S. Pat. No. 4,360,605 and U.S. Pat. No. 4,376,835.

A preferred particulate filler is a particulate glass filler selectedfrom calcium alumino silicate glass, calcium alumino fluorosilicateglass, calcium aluminumfluoroborosilicate glass, strontiumaluminosilicate glass, strontium aluminofluorosilicate glass, strontiumaluminofluoroborosilicate glass, astrontium-aluminum-sodium-fluoride-phosphorous-silicate glass, andbarium aluminum borosilicate glass, which has a median particle size(D50) of 100 to 800 nm.

The particulate filler may be a multimodal particulate fillerrepresenting a mixture of two or more particulate fractions havingdifferent average particle sizes. The particulate filler may also be amixture of particles of different chemical composition. In particular,it is possible to use a mixture of a reactive particulate material and anon-reactive particulate material.

The particulate filler used in the coating step may also comprise finalcomposite filler particles of the present invention. Specifically,composite filler particles having a small median particle size (D50) of1 to 1200 nm may be separated from a composite filler of the presentinvention and recycled into the process of the present invention inorder to provide composite filler particles having an increased medianparticle size (D50) of from 1 to 70 μm.

The surface of the particulate filler of the present invention may bemodified prior to the coating step. Accordingly, the surface modifyingagent contains a modifying compound capable of reacting with surfaceatoms of the particulate filler, thereby forming a covalent bond betweenthe surface atoms of the particulate filler and the modifying compound.Additionally, the modifying compound may contain one or morepolymerizable double bonds reactive in the crosslinking reaction afterthe particulate filler is agglomerated. The modifying agent may containone or more modifying compounds. Preferably, the modifying compoundprovides a polymerizable ligand capable of crosslinking which may be acompound of one of the following formulae (I), (II) and (III), or ahydrolysis product thereofX_(r)R_(3-r)SiL  (I)X_(r)R_(2-r)SiL′L″  (II)X_(r)SiL′L″L′″  (III)whereinX represents a hydrolyzable group;R represents an alkyl, cycloalky, cycloalkylalkyl, aralkyl or arylgroup,L, L′, L″, and L″

-   -   which may be the same or different represent independent from        each other an organic group containing one or more polymerizable        double bonds;        r is an integer of 1 to 3,        whereby the sum of X, R, L, L′, L″, and L′″ is 4 for each of        formula (I), (II), and (III).

Preferably, X is a halogen atom or OR¹, wherein R¹ is an alkyl,cycloalky, cycloalkylalkyl, aralkyl or aryl group. More preferably, R orR¹ are independently an alkyl group.

In order to impart crosslinking capability to the organofunctionalsilicon compound, L, L′, L″, and L′″ contain one or more polymerizabledouble bonds capable of taking part in a crosslinking reaction. In apreferred embodiment, L, L′, L″, and L′″ may be selected from the groupof allyl, (meth)acrylic ester groups, and (meth)acrylic amide groups.

An alkyl group may be straight-chain or branched C1-16 alkyl group,typically a C1-8 alkyl group. Examples for a C1-6 alkyl group caninclude linear or branched alkyl groups having 1 to 6 carbon atoms,preferably 1 to 4 carbon atoms, for example, methyl, ethyl, n-propyl,isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, n-pentyl, isopentyland n-hexyl. A cycloalkyl group may be a C3-16 cycloalkyl group.Examples of the cycloalkyl group can include those having 3 to 14 carbonatoms, for example, cyclopropyl, cyclobutyl, cyclopentyl and cyclohexyl.A cycloalkylalkyl group can include those having 4 to 22 carbon atoms.Examples for a cycloalkylalkyl group can include a combination of alinear or branched alkyl group having 1 to 6 carbon atoms and acycloalkyl group having 3 to 14 carbon atoms. Examples of thecycloalkylalkyl group can for example, include methylcyclopropyl,methylcyclobutyl, methylcyclopentyl, methylcyclohexyl, ethylcyclopropyl,ethylcyclobutyl, ethylcyclopentyl, ethylcyclohexyl, propylcyclopropyl,propylcyclobutyl, propylcyclopentyl, propylcyclohexyl. An aralkyl groupmay be a C7-26 aralkyl group, typically a combination of a linear orbranched alkyl group having 1 to 6 carbon atoms and an aryl group having6 to 10 carbon atoms. Specific examples of an aralkyl group are a benzylgroup or a phenylethyl group. An aryl group can include aryl groupshaving 6 to 10 carbon atoms. Examples of the aryl group are phenyl andnaphtyl.

The C1-6 alkyl group and the C3-14 cycloalkyl group may optionally besubstituted by one or more members of the group selected from a C1-4alkyl group, C1-4 alkoxy group, a phenyl group, and a hydroxy group.Examples for a C1-4 alkyl group can include linear or branched alkylgroups having 1 to 4 carbon atoms, for example, methyl, ethyl, n-propyl,isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl. Examples for anC1-4 alkoxy group can include linear or branched alkoxy groups having 1to 4 carbon atoms, for example, methoxy, ethoxy, n-propoxy, isopropoxy,n-butoxy, isobutoxy, sec-butoxy, and tert-butoxy.

Aryl groups may contain 1 to 3 substituents. Examples of suchsubstituents can include halogen atoms, C1-4 alkyl groups, C1-4 alkoxygroups, C1-4 alkylthio groups, C1-4 alkylsulfonyl groups, carboxylgroup, C2-5 alkoxycarbonyl groups, and C1-4 alkylamino groups. Here,illustrative of the halogen atoms can be fluorine, chlorine, bromine andiodine. The C1-4 alkyl groups are, for example, methyl, ethyl, n-propyl,isopropyl and n butyl. Illustrative of the C1-4 alkoxy groups are, forexample, methoxy, ethoxy and propoxy. Illustrative of the C1-4 alkylthiogroups are, for example, methylthio, ethylthio and propylthio.Illustrative of the C1-4 alkylsulfonyl groups are, for example,methylsulfonyl, ethylsulfonyl and propylsulfonyl. Illustrative of theC2-5 alkoxycarbonyl groups can be those having alkoxy groups each ofwhich contains 1 to 4 carbon atoms, for example, methoxycarbonyl, ethoxycarbonyl and propoxycarbonyl. Illustrative of the C1-8 alkylamino groupscan be those having one or two alkyl groups each of which contains 1 to4 carbon atoms, for example, methylamino, dimethylamino, ethyl amino andpropylamino. The alkyl moieties in these substituents may be linear,branched or cyclic.

A preferred particulate filler is a particulate glass filler selectedfrom calcium alumino silicate glass, calcium alumino fluorosilicateglass, calcium aluminumfluoroborosilicate glass, strontiumaluminosilicate glass, strontium aluminofluorosilicate glass, strontiumaluminofluoroborosilicate glass, and barium aluminum borosilicate glass,which has a median particle size (D50) of 200 to 800 nm, and which issurface-modified a modifying compound of one of the above formulae (I),(II) and (III), or a hydrolysis product thereof, as defined above.

The Coating Composition

The particulate filler is coated with a coating composition. The coatingcomposition contains a film-forming agent forming a coating layer on thesurface of the particulate filler said coating layer displaying reactivegroups on the surface of the coating layer, said reactive groups beingselected from addition polymerizable groups and step-growthpolymerizable groups, thereby forming a coated particulate filler. Thefilm-forming agent may form a covalent bond with functional groups onthe surface of the particulate filler. For Example, the film-formingagent may silanate the surface of the particulate filler. Alternatively,the film forming agent may adhere to the surface of the particulatefiller by non-covalent interaction such as by ionic forces or van derWaals forces. In case the film-forming agent forms a covalent bond, itis preferred that the film-forming agent carries a silyl group. In casethe film forming agent adheres based on non-covalent forces, it ispreferred that the particulate filler is modified by a modifying agent.

The coated particulate filler is reactive based on the reactive groupsand forms crosslinking groups based on a reaction between reactivegroups on particulate filler particles and optionally reactive groups ofa crosslinking agent.

The reactive groups may be addition polymerizable groups or step-growthpolymerizable groups. Addition polymerizable groups may be selected fromethylenically unsaturated groups such as (meth)acrylate groups, or vinylgroups. The step-growth polymerizable groups are selected from the groupof amino groups, hydroxyl groups, isocyanate groups, and carboxylic acidanhydride groups.

Accordingly, the film-forming agents may comprise a polymerizablemonomer which may be preferably selected from compounds characterized byone of the following formulas:

wherein X independently is O or NR2, wherein R1, R2, and R3independently represent a hydrogen atom or a C1 to C8 alkyl group whichmay be substituted; A represents a divalent substituted or unsubstitutedorganic residue having from 1 to 40 carbon atoms which may besubstituted, whereby said organic residue may contain from 1 to 6 oxygenand/or nitrogen atoms; Z represents a saturated at least trivalentsubstituted or unsubstituted C1 to C8 hydrocarbon group, a saturated atleast trivalent substituted or unsubstituted cyclic C3 to C8 hydrocarbongroup, and n is at least 3. The optionally substituted moieties R1, R2,R3, A, and Z may be substituted with from 1 to 6 acidic groups selectedfrom carboxylic acid groups, phosphate ester groups, phosphonate groups,and sulfonic acid groups.

According to a first embodiment, X is O. According to a secondembodiment, X is NR1 whereby the second embodiment provides a coatinghaving increased hydrolysis stability.

The film-forming agents may also comprise polymerizable compounds whichare selected from acrylates and methacrylates such as methyl acrylate,methyl methacrylate, ethyl acrylate, ethyl methacrylate, propylacrylate, propyl methacrylate, isopropyl acrylate, isopropylmethacrylate, 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate(HEMA), hydroxypropyl acrylate, hydroxypropyl methacrylate,tetrahydrofurfuryl acrylate, tetrahydrofurfuryl methacrylate, glycidylacrylate, glycidyl methacrylate, the diglycidyl methacrylate ofbis-phenol A (“bis-GMA”), glycerol mono- and di-acrylate, glycerol mono-and di-methacrylate, ethyleneglycol diacrylate, ethyleneglycoldimethacrylate, polyethyleneglycol diacrylate (where the number ofrepeating ethylene oxide units vary from 2 to 30), polyethyleneglycoldimethacrylate (where the number of repeating ethylene oxide units varyfrom 2 to 30 especially triethylene glycol dimethacrylate (“TEGDMA”),neopentyl glycol diacrylate, neopentylglycol dimethacrylate,trimethylolpropane triacrylate, trimethylol propane trimethacrylate,mono-, di-, tri-, and tetra-acrylates and methacrylates ofpentaerythritol and dipentaerythritol, 1,3-butanediol diacrylate,1,3-butanediol dimethacrylate, 1,4-butanedioldiacrylate, 1,4-butanedioldimethacrylate, 1,6-hexane diol diacrylate, 1,6-hexanedioldimethacrylate, di-2-methacryloyloxethyl hexamethylene dicarbamate,di-2-methacryloyloxyethyl trimethylhexanethylene dicarbamate,di-2-methacryloyl oxyethyl dimethylbenzene dicarbamate,methylene-bis-2-methacryloxyethyl-4-cyclohexyl carbamate,di-2-methacryloxyethyl-dimethylcyclohexane dicarbamate,methylene-bis-2-methacryloxyethyl-4-cyclohexyl carbamate,di-1-methyl-2-methacryloxyethyl-trimethyl-hexamethylene dicarbamate,di-1-methyl-2-methacryloxyethyl-dimethyl benzene dicarbamate,di-1-methyl-2-methacryloxyethyl-dimethylcyclohexane dicarbamate,methylene-bis-1-methyl-2-methacryloxyethyl-4-cyclohexyl carbamate,di-1-chloromethyl-2-methacryloxyethyl-hexamethylene dicarbamate,di-1-chloromethyl-2-methacryloxyethyl-trimethylhexamethylenedicarbamate, di-1-chloromethyl-2-methacryloxyethyl-dimethylbenzenedicarbamate, di-1-chloromethyl-2-methacryloxyethyl-dimethylcyclohexanedicarbamate, methylene-bis-2-methacryloxyethyl-4-cyclohexyl carbamate,di-1-methyl-2-methacryloxyethyl-hexamethylene dicarbamate,di-1-methyl-2-methacryloxyethyl-trimethylhexamethylene dicarbamate,di-1-methyl-2-methacryloxyethyl-dimethylbenzene dicarbamate,di-1-methyl-2-metha-cryloxyethyl-dimethylcyclohexane dicarbamate,methylene-bis-1-methyl-2-methacryloxyethyl-4-cyclohexyl carbamate,di-1-chloromethyl-2-methacryloxyethyl-hexamethylene dicarbamate,di-1-chloromethyl-2-methacryloxyethyl-trimethylhexamethylenedicarbamate, di-1-chloromethyl-2-methacryloxyethyl-dimethylbenzenedicarbamate, di-1-chloromethyl-2-methacryloxyethyl-dimethylcyclohexanedicarbamate,methylene-bis-1-chloromethyl-2-methacryloxyethyl4-cyclohexyl carbamate,2,2′-bis(4-methacryloxyphenyl)propane, 2,2′bis(4-acryloxyphenyl)propane,2,2′-bis[4(2-hydroxy-3-methacryloxy-phenyl)]propane,2,2′-bis[4(2-hydroxy-3-acryloxy-phenyl)propane,2,2′-bis(4-methacryloxyethoxyphenyl)propane,2,2′-bis(4-acryloxyethoxyphenyl)propane,2,2′-bis(4-methacryloxypropoxyphenyl)propane,2,2′-bis(4-acryloxypropoxyphenyl)propane,2,2′-bis(4-methacryloxydiethoxyphenyl)propane,2,2′-bis(4-acryloxydiethoxyphenyl)propane,2,2′-bis[3(4-phenoxy)-2-hydroxypropane-1-methacrylate]propane, and2,2′-bis[3(4-phenoxy)-2-hydroxypropane-1-acryalte]propane, may bementioned. Other suitable examples of polymerizable components areisopropenyl oxazoline, vinyl azalactone, vinyl pyrrolidone, styrene,divinylbenzene, urethane acrylates or methacrylates, epoxy acrylates ormethacrylates and polyol acrylates or methacrylates. Preferably, thepolymerizable compound has a molecular weight of at most 10,000 Da, morepreferably at most 8000 Da, and still mor preferably, 5000 Da.

Step-growth polymerizable groups may be selected from groups capable ofundergoing condensation reactions for forming linkages such as urethane,ether, amine, amide or ester linkages. Alternatively, step-growthpolymerizable groups may be selected from groups undergoing additionreactions for forming linkages such as amine linkages by e.g. a Michaeladdition.

A preferred class of film-forming agents forming a coating layer on thesurface of the particulate filler are amino group containing silanes orsiloxanes suitable for crosslinking with isocyanates, isocyanate/aminemixtures, isocyanate/diol mixtures, epoxides, epoxide/amine mixtures,anhydrides, carboxylic acids such as polyacrylic acid.

The amino group containing silanes or siloxanes may be selected from3-aminopropyltriethoxysilane (APTES), 3-aminopropyltrimethoxysilane,m-aminophenyltrimethoxysilane, p-aminophenyltrimethoxysilane,3-aminopropyltris(methoxyethoxyethoxy)silane,3-(m-aminophenoxy)propyltrimethoxysilane,3-Aminopropylmethyldiethoxysilane, 3-aminopropyldimethylethoxysilane,N-(2-aminoethyl)-3-aminopropyltrimetoxysilane,aminopropylmethyldimethoxysilane, aminopropyldimethylmethoxysilaneaminopropylmethyldiethoxysilane, aminopropyldimethylethoxysilane,2-(aminoethyl)-3-aminopropyltrimethoxysilane (AEPTMS),2-(aminoethyl)-3-aminopropyldimethoxymethylsilane,2-(aminoethyl)-3-aminopropyldimethylmethoxysilane,2-(aminoethyl)-3-aminopropyltriethoxysilane,2-(aminoethyl)-3-aminopropyldiethoxymethylsilan,2-(aminoethyl)-3-aminopropyldimethylethoxysilane,(3-trimethoxysilylpropyl)diethylenetriamine(3-dimethoxymethylsilylpropyl)diethylenetriamine,(3-dimethylmethoxysilylpropyl)diethylenetriamine,(3-triethoxysilylpropyl)diethylenetriamine (TMSPDETA),(3-diethoxymethylsilylpropyl)diethylenetriamine,(3-dimethylethoxysilylpropyl)diethylenetriamine.

The compounds may be used alone or in combination of two or moredifferent compounds.

A further preferred class of film-forming agents forming a coating layeron the surface of the particulate filler are epoxide group containingsilanes or siloxanes suitable for crosslinking with amines, mixedepoxide-amine compounds, or carboxylic acids such as polyacrylic acid.

Preferred compounds are selected from 2-(3,4-epoxycyclohexyl)ethyltriethoxy silane, 2-(3,4-epoxycyclohexyl)ethyl trimethoxy silane,(3-glycidoxypropyl)trimethoxy silane, (3-glycidoxypropyl)triethoxysilane, 5,6-epoxyhexyl triethoxy silane,(3-glycidoxypropyl)methyldiethoxy silane,(3-glycidoxypropyl)methyldimethoxy silane, and(3-glycidoxypropyl)dimethylethoxy silane.

A further preferred class of film-forming agents forming a coating layeron the surface of the particulate filler are isocyanate group containingsilanes or siloxanes suitable for crosslinking with amines,isocyanate-amine mixtures, or isocyanate-diol mixtures. A preferredcompound is 3-isocyanatopropyltriethoxy silane.

A further preferred class of film-forming agents forming a coating layeron the surface of the particulate filler are carboxylic acid anhydridegroup containing silanes or siloxanes suitable for crosslinking withamines, diols, anhydride-amine mixtures, and anhydride-diol mixtures. Apreferred compound 3-(triethoxysilyl)propylsuccinic anhydride.

Moreover, the film-forming agent may comprise organically modifiedceramic particles (ORMOCER) as disclosed in EP0451709.

The coating step may be carried out by mixing the film-forming agentwith the dispersed particulate filler, optionally in the presence of asuitable solvent. The solvent is not particularly limited as long as thefilm-forming agent may be dissolved therein. Examples of suitablesolvents are selected from tetrahydrofurane, 1,4-dioxane, acetone,ethanol, propanol, pentane, hexane, heptane, cyclohexane, toluene,xylene, chloroform, methylene chloride, methyl ethyl ketone, methylisobutyl ketone, diethyl ether, tert.-butylmethyl and ether. Thesolvents may used alone or as a mixture of two or more liquids inamounts which are miscible with each other.

The coating composition may preferably contain film-forming compoundscompounds in an amount of from 5 to 100 wt-%, preferably in an amount offrom 20 to 70 wt. % based on the entire composition. According to aspecific embodiment, the coating composition contains the film-formingcompounds in an amount of 100 wt. %.

Preferably, the polymerizable composition has a dynamic viscosity in therange of from 0.0001 to 15 Pas (23° C.). Preferably, the thickness ofthe coating layer of the polymerizable monomer is less than the medianparticle diameter (D50) of the particulate filler.

The Coating of the Particulate Filler

The coating temperature is not particularly limited. Accordingly, thecoating step may in general be carried out at a temperature of from −20°C. 250° C. In particular, the coating temperature may be in the rangebetween 0° C. to below the boiling point of the coating composition.Preferably, the dispersion step is carried out at a temperature in therange of from 10° C. to 150° C. The dispersion may be carried out for upto 10 hours, preferably from 10 seconds to 1 hour.

High shear forces may be applied by intense mechanical stirring,ultrasonication or atomization by using a nozzle.

Subsequent to the coating step, the coated particulate filler may beisolated. The isolation may be carried out by separation of the coatedparticulate filler by using centrifugation or filtration.

Alternatively, any volatile components including any solvent may beseparated by evaporation. The coated particulate filler may be furtherpurified by drying at an elevated temperature of from 50° C. to 150° C.for 1 hour to 36 hours. Alternatively, the coated particles areagglomerated concurrently for forming a granulation of the coatedparticulate filler.

In order to eliminate particles having an undesired particle size, thecoated particulate filler may be sieved prior to the agglomeration step.

The Step of Agglomerating the Coated Particulate Filler

Subsequent to or concurrent with the coating of the particulate filler,the coated particulate filler is agglomerated for providing agranulation of the coated particulate filler.

Agglomeration means that the fine coated particulate filler is dispersedeither in a gas or a liquid for forming particles having a larger medianparticle diameter (D50) as a coarser product. The process ofagglomeration is not particularly limited as long as the particle sizeof the granulate may be controlled by the agglomeration process.

Agglomeration may be carried out by spray agglomeration (Spray Methods),wherein a suspension of the coated particulate filler is atomized, andthe liquid is evaporated from the droplets by means of hot air, as apreliminary drying step. The first cohesive forces are the capillaryforces, which are followed by crystal (fluid) bridges at the contactpoints. Advantageously, spray agglomeration is carried out for preparingcomposite filler particles having a relatively small particle size.

Agglomeration may be carried out by growth agglomeration wherein fineparticles are brought into contact with each other in a flowing systemor in air, optionally in the presence of an additional liquid bindersuch as a crosslinking agent. Growth agglomeration is the growth of moreor less solid agglomerates in either of two environments. The first is arotating apparatus that produces both a mixing and a rolling motion. Thesecond is a turbulently agitated suspension of particles that generatesinterparticle collisions. There is a stable accumulation if theattractive forces in the system always are greater than the destructiveforces present in the system. The process may be carried out withequipment selected from inclined drums, cones, pans, paddle mixers, andplowshare mixers. Furthermore, agglomeration may also involve the use ofa fluidized bed. Advantageously, growth agglomeration is carried out forpreparing composite filler particles having a relatively large particlesize.

During agglomeration, fluid material bridges between the coatings formedby the polymerizable composition on the surface of the particulatefiller are formed. Fluid bridges may be formed by capillary forcesbetween the particle and the polymerizable composition and optionally afurther binder such as a crosslinking agent.

In case the agglomeration step is carried out in a liquid dispersionmedium, the agglomeration may be carried out at a temperature of from 0°C. to the boiling point of the liquid. Preferably, the dispersion stepis carried out at a temperature in the range of from 10° C. to 150° C.

The agglomeration step may be carried out by mixing the coatedparticulate filler optionally in the presence of a further crosslinkingagent and optionally in the presence of a further particulate filler notdisplaying reactive groups, the presence of a suitable solvent. Thesolvent is not particularly limited as long as a crosslinking agent maybe dispersed or dissolved therein. Examples of suitable solvents areselected from tetrahydrofurane, 1,4-dioxane, acetone, ethanol, propanol,pentane, hexane, heptane, cyclohexane, toluene, xylene, chloroform,methylene chloride, methyl ethyl ketone, methyl isobutyl ketone, diethylether, tert.-butylmethyl and ether. The solvents may used alone or as amixture of two or more liquids in amounts which are miscible with eachother.

The optional further particulate filler not displaying reactive groupsmay be selected from any of the particlulate fillers or modifiedparticulate fillers, vide supra.

The optional further crosslinking agent may be selected from any of thefilm-forming agents useful for coating the particulate filler ormodified particulate filler, vide supra.

In case the agglomeration step is carried out in a gaseous dispersionmedium, the agglomeration may be carried out at a temperature of from−20° C. 250° C. 0° C., preferable at a temperature of from 0° C. to 100°C. More preferably, the dispersion step is carried out at a temperaturein the range of from 10° C. to 40° C.

The agglomeration may be carried out for up to 10 hours, preferably from10 seconds to 1 hour. In order to complete the crosslinking reaction,the agglomerated coated filler may be heated to an elevated temperature,preferably after the solvent has been evaporated. The composite fillerparticles are prepared according to the present invention by a processcomprising a further step of crosslinking the granulation of the coatedparticulate filler for providing the composite filler particles.

Crosslinking may additionally be carried out be irradiating and/orheating the granulate. Irradiation maybe carried out by subjecting thegranulate to radiation having a wavelength in the range of from 100 to1000 nm for 1 to 60 minutes. Heating may be carried out by subjectingthe granulate to a temperature of from 30 to 250° C.

Subsequent to the agglomeration step, the composite filler particles maybe isolated. The isolation may be carried out by separation of thecoated particulate filler by using centrifugation or filtration.Alternatively, any volatile components including any solvent may beseparated by evaporation. The coated particulate filler may be furtherpurified by drying at an elevated temperature of from 50° C. to 150° C.for 1 hour to 36 hours.

In order to adjust the particle size of the composite filler particles,the composite filler particles may be dispersed in a suitable dispersionfluid by using high shear forces such as mechanical comminution, mixing,ultrasonication, or atomization using a nozzle.

In order to eliminate particles having an undesired particle size, thecoated particulate filler may be sieved prior to the agglomeration step.

The crosslinking reaction may be a chain growth polymerization and/or astep growth polymerization.

When crosslinking is carried out by chain growth polymerization,unsaturated double bonds present on the coated particulate filler in thegranules react by a mechanism selected from a free radical mechanism,cationic addition polymerization and anionic addition polymerization.Accordingly, the polymerizable composition may contain a polymerizationinitiator and a stabilizer. Suitable radical polymerization initiatorsmay be selected from the following classes of initiator systems:

Combinations of an organic peroxide and an amine, wherein the organicperoxide may be benzoyl peroxide or a thermally more stable peroxidesuch as 2,5-dimethyl-2,5-di(benzolyperoxy)hexane, tert.-butylamylperoxide, di-(tert.-butyl) peroxide, cumene hydroperoxide,tert.-butylhydroperoxide, tert.butyl-peroxy-(3,5,5-trimethyl hexanoate),tert.-butylperoxy benzoate and tert.butylperoxy-2-ethylhexyl carbonate.The amine compound may be an aromatic amine compound such as DMABE.

Combinations of an organic peroxide, a reducing agent and a suitablemetal ion. The peroxide may be selected from benzoyl peroxide,2,5-dimethyl-2,5-di(benzolyperoxy)hexane, tert.-butylamyl peroxide,di-(tert.-butyl) peroxide, cumene hydroperoxide,tert.-butylhydroperoxide, tert.butyl-peroxy-(3,5,5-trimethyl hexanoate),tert.-butylperoxy benzoate and tert.butylperoxy-2-ethylhexyl carbonate.The reducing agent may be a protected reducing agent in inactive form,which forms an active reducing agent as disclosed in EP 0 951 894. Themetal ion may be a salt of a metal or an organometalic compound, whichmay be present as an acetate, salicylate, naphenoate, thiourea complex,acetylacetonate or ethylene tetramine acidic acid. Suitable metal ionsare selected from copper, iron, and silver.

Combinations of a hydroperoxide and a metal ion. A suitablehydroperoxide is hydrogen peroxide. A suitable metal may be selectedfrom iron and copper.

Transition metal carbonyl compounds such as dicopper octacarbonylcomplexes which may from radical species.

Alkylboron compounds such as alkyl boranes.

Combinations of peroxdisulphate salts and thiol compounds.

It is also possible to use a photoinitiator such as a camphorquinone/amine intiator.

When crosslinking is carried out by a step-growth polymerisation, abridge between the coated particulate filler in the granules is formedby the stepwise reaction between functional groups of monomers such asby a condensation reaction or the formation of a urethane bond.

The particulate filler is agglomerated, optionally in the presence of afurther crosslinking agent and optionally in the presence of a furtherparticulate filler not displaying reactive groups, for providing agranulation of the coated particulate filler wherein the granulationcontains the coated particulate filler particles separated from andconnected to each other by at least one coating layer, and whereby thecoating layers may be crosslinked by crosslinking groups obtained byreacting the reactive groups and optionally a further crosslinking agent

The coated particulate filler may be crosslinked by using anhydridegroup containing crosslinking agents for crosslinking coated particulatefiller particles displaying amine or hydroxyl groups on the coatinglayer.

A preferred group of anhydride group containing crosslinking agents areselected from 2,2-bis-(4-phthalic anhydride-4-oxyphenyl)-propane,butantetracarboxylic acid dianhydride, 4,4′-oxybis-phthalic acidanhydride, benzophenone-3,3′,4,4′-tetracarboxylic acid dianhydride,biphenyl-3,3′,4,4′-tetracarboxylic acid dianhydride, pyromellitic aciddianhydride, poly(ethylene-alt-maleic acid anhydride).

A further class of crosslinking agent is a polyisocyanate crosslinkingagent for crosslinking coated particulate filler particles displayingamine or hydroxyl groups on the coating layer. Preferred polyisocyanatecrosslinking agents are selected from the group of1,3-bis-(1-isocyanato-1-methylethyl)benzene,1,3-bis-(isocyanatomethyl)-cyclohexane,hexamethylene diisocyanate,toluene-2,4-diisocyanate, trimethylhexamethylene diisocyanate, methylenedi(phenylisocyanate), 4,4′-diisocyanatodicyclohexyl methane, andisophorone diisocyanate.

A further class of crosslinking agents are epoxide crosslinking agentsfor crosslinking coated particulate filler particles displaying aminegroups on the coating layer. Preferred epoxide crosslinking agents areselected from 1,4-cyclohexane dimethanol-diglycidyl-ether,1,4-butanedioldiglycidyl ether, bisphenol-F diglycidyl ether,isocyanuric acid tris-(2,3-epoxypropyl) ester, neopentylglycoldiglycidyl ether, triphenylolmethan triglycidyl ether, and bisphenol-Adiglycidyl ether.

A further class of crosslinking agents are amine crosslinking agents forcrosslinking coated particulate filler particles displaying isocyanategroups, epoxide groups, or anhydride groups on the coating layer.Preferred amine crosslinking agents are selected from ethylene diamine,1,3-propane diamine, diethylene triamine, triethylene tetraamine, andtetraethylene pentamine, aminoethyl piperazine, polyether amines such as4,7,10-trioxa-1,13-tridecane diamine 2,2′-ethylendioxy) diethylamine,1,3-bis-(aminomethyl) cyclohexane, 1,3-bis-(4-aminophenoxy)benzene,4,4′-methylene bis-cyclohexylamine,5-amino-1,3,3-trimethylcyclohexanemethylamine,3,(4),8,(9)-bis(aminomethyl)-tricyclo-5.2.1.0 (2,6)-decane.

A further class of crosslinking agents are hydroxyl group containingcrosslinking agents for crosslinking coated particulate filler particlesdisplaying isocyanate groups, epoxide groups and anhydride groups.Preferred hydroxyl group containing crosslinking agents are selectedfrom polyols (e.g. Desmophen® Polyetherpolyol), 1,3-propane diol,ethylene glycol, diethylene glycol, triethylene glycol.

The above crosslinking agents may be used alone or in combination.

Now the present invention will be described to generic embodiments.

FIG. 1 is a flow chart illustrating a first generic embodiment of theprocess for the preparation of a composite filler particles of thepresent invention based on a suspension/emulsion technique. Accordingly,the surface of a particulate filler is modified prior to the coatingstep by silanation. A suitable silanating agent is 3-methacryloylpropyltrimethoxysilane. Subsequently, the particulate filler is coated with acoating composition containing a film-forming agent forming a coatinglayer on the surface of the particulate filler. A suitable coatingcomposition may contain a silanation agent having reactive groups suchas amino groups, carboxylic acid anhydride groups, isocyanate groups orepoxy groups. The coating composition may optionally contain a reactivediluent and or a solvent. Accordingly, a coated particular filler isprovided wherein said coating layer displays reactive groups on thesurface of the coating layer. Subsequently, the coated particulatefiller is agglomerated by emulsification or suspension of the mixture ina solvent wherein neither the particles nor the coating composition aresoluble. Accordingly, a granulation of the coated particulate filler isprovided wherein the granulation contains the coated particulate fillerparticles and the optional further particulate filler particlesseparated from and connected to each other by at least one coatinglayer. Subsequently, polymerization and crosslinking of the reactivegroups displayed on the coated particles is carried out, whereby the atleast one coating layer is crosslinked by crosslinking groups obtainedby reacting the reactive groups the coated particulate filler. Accordingto the first generic embodiment, composite filler particles may beprovided which have a median particle size (D50) of from 1 to 70 μm,wherein reactive groups are transformed into crosslinking groupsobtained by reacting reactive groups and optionally a furthercrosslinking agent, and wherein the particulate filler is the maincomponent by volume of the composite filler particles.

FIG. 2 is a flow chart illustrating a second generic embodiment of theprocess for the preparation of composite filler particles of the presentinvention wherein a spray dryer is used. Accordingly, the surface of aparticulate filler is modified prior to the coating step by silanation.A suitable silanating agent is 3-methacryloylpropyl trimethoxysilane.Subsequently, the modified particles are mixed with a coatingcomposition whereby a dispersion of the modified particles is obtained.The dispersion of the modified articles in the coating composition isthen spray dried by using a spray dryer whereby the particles areagglomerated. Accordingly, the coating of the particulate filler and theagglomeration of the coated particulate filler takes place concurrently.The spray dried product may be heat treated for reacting the reactivegroups and optionally a further crosslinking agent and to providecrosslinking groups. According to the second generic embodiment,composite filler particles may be provided which have a median particlesize (D50) of from 1 to 70 μm, wherein reactive groups are transformedinto crosslinking groups obtained by reacting reactive groups andoptionally a further crosslinking agent, and wherein the particulatefiller is the main component by volume of the composite fillerparticles.

FIG. 3 is a flow chart illustrating a third generic embodiment of theprocess of the present invention based on wherein a high shear mixer ora fluidized bed is used. Accordingly, the surface of a particulatefiller is modified prior to the coating step by silanation. A suitablesilanating agent is 3-methacryloylpropyl trimethoxysilane. Subsequently,the particulate filler is coated with a coating composition containing afilm-forming agent forming a coating layer on the surface of theparticulate filler and concurrently agglomerated in a high shear mixeror a fluidized bed. A suitable coating composition may contain asilanation agent having reactive groups such as amino groups, carboxylicacid anhydride groups, isocyanate groups or epoxy groups. The coatingcomposition may optionally contain a reactive diluent and or a solvent.Accordingly, an agglomerated coated particular filler is providedwherein said coating layer displays reactive groups on the surface ofthe coating layer, and wherein the coated particulate filler isagglomerated. The granulation of the coated particulate filler isprovided wherein the granulation contains the coated particulate fillerparticles and the optional further particulate filler particlesseparated from and connected to each other by at least one coatinglayer. Subsequently, polymerization and crosslinking of the reactivegroups displayed on the coated particles is carried out, whereby the atleast one coating layer is crosslinked by crosslinking groups obtainedby reacting the reactive groups the coated particulate filler. Accordingto the third generic embodiment composite filler particles may beprovided which have a median particle size (D50) of from 1 to 70 μm,wherein reactive groups are transformed into crosslinking groupsobtained by reacting reactive groups and optionally a furthercrosslinking agent, and wherein the particulate filler is the maincomponent by volume of the composite filler particles.

The Dental Composition

The composite filler particles of the present invention may be used forthe preparation of a dental composition. A dental composition ispreferably a dental restorative material. The dental restorativematerial may be selected from a dental composite, a dental cement or aresin reinforced dental cement. A dental composite may be a highlyfilled dental composite, a flowable composite, a compomer, a root canalsealer, or a pit and fissure sealant. A dental cement may be a glassionomer cement or a luting cement.

A dental composite contains the particulate composite filler of thepresent invention and a polymerizable monomer, a polymerizationinitiator, and optionally an additional filler. According to a preferredembodiment, the additional filler may be a particulate filler orsurface-modified particulate filler as used for the preparation of thecomposite particulate filler of the present invention.

In case of a particulate composite filler for use in a dental cement,the polymerizable compound may also be a modified polyacid havingpolymerizable double bonds.

It is possible to use a combination of both types of polymerisations forproviding a resin reinforced dental cement.

The polymerizable monomer is preferable a compound having at least onepolymerizable group. Generally in dental compositions, radicalpolymerization is performed. Therefore, the polymerizable group istypically a radical polymerizable group. As the polymerizable group,(meth)acrylolylamino or a (meth)acryloyloxy group, is preferable. Thepolymerizable monomers may be selected from the same polymerisablecompounds as contained in the polymerisable composition used for coatingthe particulate filler.

The additional filler includes glass particles such as bariumaluminum-borosilicate glass, barium aluminofluorosilicate glass andmixtures thereof. In these materials, barium can also be substituted bystrontium, and may also contain fluoride. Other useful materials includecalcium hydroxy ceramics, and others such as those fillers disclosed inU.S. Pat. Nos. 5,338,773, 5,710,194, 4,758,612, 5,079,277, and4,814,362. These materials may have any morphology or shape, includingspheres, regular or irregular shapes, filaments or whiskers, and thelike and silane treated (silane coupled) or provided with othertreatments as is conventional for dental fillers.

The initiator may be any thermal initiator or photoinitiatorconventionally used in the dental field. A dental composite may,furthermore, contain inhibitors, UV absorbers, accelerators, orfluorescing agents.

A dental cement is usually powder liquid systems consisting of linearpoly(alkenoic acid)s and reactive ion releasing active glasses. The mostcommon poly(alkenoic acid)s are polymers such as polyacrylic acid orcopolymers of acrylic and itaconic acid, acrylic acid and maleic acidand to some degree a copolymer of acrylic acid with methacrylic acid. Inthe presence of water, the poly(alkenoic acid) attacks the glass powderwhereby metal ions such as calcium, aluminum and strontium are releasedunder formation of intra- and intermolecular salt bridges whichcrosslink the composition. The particulate composite filler of thepresent invention may be incorporated into dental cement either as anunreactive glass filler or as a reactive glass filler.

EXAMPLES

The present invention will now be explained in further detail withreference to the following examples.

According to the present invention, the median particle size (D50) ofthe composite filler particles is measured according to the followingprocedure:

2 g of the particles are added to 4 mL of ethanol and 2 drops of Tween85® are added. The particles are pre-dispersed by shaking the mixtureuntil a visibly homogeneous mixture is achieved. Subsequently, themixture is added into the measuring cell of a Malvern Mastersizer S,containing 800 mL of water and being equipped with a stirrer set to 2200and an ultrasound probe set to 1800, while stirring the dispersion untilan turbidity of approx. 20-25% was reached. The median particle size(D50) is measured after applying ultrasound from the ultrasound probe inthe measurement cell under stirring for 10 Minutes. Ultrasound isapplied to break up loosely aggregated particles and to distinguish themfrom the agglomerated particles.

Example 1—Coating of a Particulate Filler

112.5 g of a strontium-aluminum-sodium-fluoride-phosphorous-silicateglass having a particle size D50=0.7 μm are dispersed in 200 mL ethanolat room temperature for 15 minutes in an ultrasound bath. 1.6 mL (1.6 g)(3-aminopropyl)trimethoxysilane dissolved in 10 mL ethanol, and 1.5 mL(1.4 g) 3-(trimethoxysilyl)propylmethacrylat, dissolved in 10 mL ethanolare added simultaneously to the glass suspension. During the addition,suspension is treated with ultrasound. After the addition is complete,the suspension is treated for additional 30 minutes with ultrasound atroom temperature. Subsequently, the solvent is removed in vacuo and theresidue is dried for 24 h at 100° C. for providing a coated particulatefiller. The coated particulate filler is the sieved by using a 180 μmsieve. The median particle size D50 of the coated particulate filler wasdetermined to be D50=0.74 μm.

In a beaker containing about 50 mL water, a portion of about 50 mg ofthe coated particulate filler is placed on the surface whereby thecoated particulate filler stays afloat, which indicates that theparticulate filler has been coated with hydrophobic3-(trimethoxysilyl)propyl methacrylate.

20 mg of the coated particulate filler are placed on a thin layerchromatography plate and treated with a solution of ninhydrin (0.5 gninhydrin in 100 mL ethanol) and heated whereby a blue color appearswhich indicates the presence of amino groups.

Example 2—Aggregation of Coated Particulate Filler

70 mg isophoren diamine (5-amino-1,3,3-trimethylcyclohexanemethylamine)are dissolved in 2.5 mL tetrahydrofuran (THF). 280 mgbisphenol-A-diglycidyl ether are dissolved in 2.5 mL THF. Both solutionsare mixed with 5.00 g of the coated particulate filler and the solventis removed in vacuo (50 mbar) at 30° C. The resulting granulation of thecoated particulate filler is kept in a sealed container for 5 hours at70° C. Subsequently, about 1.5 g of the resulting granulation is addedto 10 mL of ethanol and treated for 1 h with ultrasound. The medianparticle size of the composite filler particles was determined to beD50=3.3 μm.

Example 3—Aggregation of Coated Particulate Filler

77 mg TCD-diamin (3(4),8(9)-Bis-(aminomethyl)-tricyclo[5.2.1.02,6]decan)are dissolved in 2.5 mL THF. 280 mg bisphenol-A-diglycidyl ether aredissolved in 2.5 mL THF. Both solutions are mixed with 5.00 g of thecoated particulate filler and the solvent is removed in vacuo (50 mbar)at 30° C. The resulting granulation of the coated particulate filler iskept in a sealed container for 5 hours at 70° C. Subsequently, about 1.5g of the resulting granulation is added to 10 mL of ethanol and treatedfor 1 h with ultrasound. The median particle size of the compositefiller particles was determined to be D50=4.5 μm.

Example 4—Agglomeration Using a Büchi Mini Spray Dryer (BüchiMinispruhtrockner B-290)

100 g of modified particles (median particle size D50=1.2 μm) (modifiedwith 1.5 wt.-% 3-Methacryloylpropyltrimethoxysilane and 1.5 wt.-%3-Aminopropyltrimethoxysilane) were mixed with a defined amount ofacetone (see table 1) using a magnetic stirrer. Epilox (CAS 1675-54-3)and TCD diamine (CAS 68889-71-4) (see table 1) were added to thismixture. The mixture was then injected into the spray dryer using itsinternal peristaltic pump and a 2 component jet nozzle from Büchi(nozzle diameter=1.4 mm) (pump rate see table 1). The aspiratorefficiency was set to 100%. The process parameters used are listed intable 1.

After the agglomerated particles were spray dried, the agglomeratedparticles were sieved through a 300 μm sieve and dried for 24 hours at80° C. and the median particle size (D50) measured according to theprocedure described before. The median particle sizes (D50) of theagglomerated particles are listed in table 1 below.

TABLE 1 List of variable amounts of Epilox and TCD diamine - variableprocess parameters and resulting median particle size (D50) Medianparticle size of Amount Amount Amount Air flow Pump Inlet agglomeratedExample epilox TCD acetone (Rotameter) rate temperature particles(Experiment) g g g mm % ° C. (D50) μm 4-1 8.44 1.56 65.20 50 40 130 15.2SNO-01-15-01 4-2 12.35 2.65 78.60 40 20 100 20.0 SNO-01-16-01 4-3 12.352.65 54.42 40 20 160 36.4 SNO-01-19-01 4-4 4.53 0.47 80.26 60 20 100 8.8SNO-01-20-01 4-5 12.35 2.65 78.60 60 60 100 52.6 SNO-01-27-01

Example 5—Agglomeration Using a High Shear Mixer (Diosna LaboratoryMixer P1-6)

450 g of modified particles (median particle size D50=1.2 μm) (modifiedwith 1.5 wt.-% 3-Methacryloylpropyltrimethoxysilane and 1.5 wt.-% 3-Aminopropyltrimethoxysilane) were placed into the mixer. Separately, adefined amount of Epilox (CAS 1675-54-3) was dissolved in acetone (seetable 2) and TCD diamine (68889-71-4) added to this mixture (see table2). This mixture was then injected into the mixer using a 2 componentSchlick jet nozzle (model 970/7, nozzle size 0.8 mm) and a Watson MarlowD323 peristaltic pump. During the injection process, the particles weremixed using a 3 wing impeller and a chopper at defined rpm (see table2). The pressure applied to the jet nozzle as well as the pump speed inrpm is listed in table 2. After the complete addition, the obtainedpowder might be subjected to an additional mixing step (see table 2).After the mixing was complete, the agglomerated particles were sievedthrough a 300 μm sieve and dried for 24 hours at 80° C. and the medianparticle size (D50) measured according to the procedure describedbefore. The median particle sizes (D50) of the agglomerated particlesare listed in Table 2 below.

TABLE 2 List of variable amounts of Epilox and TCD diamine - variableprocess parameters and resulting median particle size (D50) MedianPressure particle applied size of Amount Amount Amount Impeller Chopperto jet Additional Pump agglomerated Example epilox TCD acetone speedspeed nozzle mixing speed particles (Experiment) g g g rpm rpm bar secrpm (D50) μm 5-1 37.99 7.01 45.00 500 1600 0.5 60 38 45.3 KJ18-182-2 5-220.39 2.11 22.50 250 2200 0.7 120 17 11.3 KJ-18-182-15 5-3 20.39 2.1122.50 750 1000 0.7 120 17 5.7 KJ-18-182-11 5-4 20.39 2.11 22.50 250 10000.3 120 17 12.7 KJ-18-182-18 5-5 20.39 2.11 22.50 750 1000 0.3 0 17 6.2KJ-18-182-14

Example 6—Agglomeration Using a High Shear Mixer (Diosna LaboratoryMixer P1-6)

450 g of modified particles (median particle size D50=1.0 μm) (modifiedwith 3.1 wt.-% 3-Methacryloylpropyltrimethoxysilane) were placed intothe mixer. Separately, a defined amount of ethoxylatedbisphenol-A-dimethacrylate (EBA) (CAS 41637-38-1) was mixed with adefined amount of trimethylolpropane trimethacrylate (TMPTMA)(3290-92-4) and a defined amount of tert-butylperoxy2-ethylhexylcarbonate (TBPEHC) (CAS 34443-12-4) (see table 3) using amagnetic stirrer until a visually homogeneous mixture was achieved. Thismixture was then injected into the mixer using a 2 component Schlick jetnozzle (model 970/7, nozzle size 0.8 mm) and a Watson Marlow D323peristaltic pump. During the injection process, the particles were mixedusing a 3 wing impeller and a chopper at defined rpm (see table 3). Thepressure applied to the jet nozzle was set to 0.3 bar, while the appliedpump speed in rpm is listed in table 3. After the complete addition, theobtained powder was subjected to an additional mixing step for 60seconds using the same parameters for the impeller and chopper rpm.After the mixing was complete, the agglomerated particles were sievedthrough a 300 μm sieve and dried for 24 hours at 80° C. and the medianparticle size (D50) measured according to the procedure describedbefore. The median particle sizes (D50) of the agglomerated particlesare listed in table 3 below.

TABLE 3 List of variable amounts of EBA, TMPTMA and TBPEHC - variableprocess parameters and resulting median particle size (D50) Medianparticle size of Amount Amount Amount Impeller Chopper Pump agglomeratedExample EBA TMPTMA TBPEHC speed speed speed particles (Experiment) g g grpm rpm rpm (D50) μm 6-1 15.75 15.75 0.50 250 1500 20 3.1 SST-02-33-016-2 33.75 11.25 0.90 200 2000 15 11.5 SST-02-33-02 6-3 10.13 3.38 0.27150 2000 15 2.2 SST-02-34-01 6-4 15.75 15.75 0.50 200 2000 20 3.4SST-02-34-02 6-5 23.63 7.88 1.00 200 2000 20 4.1 SST-02-35-02

Example 7—Measurement of the Median Particle Size (d₅₀) of theAgglomerated Particles

2 g of the agglomerated particles were added to 4 mL of Ethanol andoptionally 2 drops of Tween 85 were added. The agglomerated particleswere pre-dispersed by shaking this mixture until a visible homogeneousmixture was achieved. This mixture was then added into the measuringcell of a Malvern Mastersizer S, containing 800 mL of water and beingequipped with a stirrer set to 2200 and an ultrasound probe set to 1800,while stirring the dispersion until an obscuration of approx. 20-25% wasreached. The median particle size (d₅₀) was measured after havingapplied ultrasound from the ultrasound probe in the measurement cellunder stirring for 10 Minutes. Ultrasound was applied to deaggregateloosely aggregated particles and to distinguish them from theagglomerated particles (particles cross linked by added monomers).

Example 8—Determination of the Degree of Polymerization of the Monomersby Measuring the Amount of Extractable Monomers

Method 1:

Up to 1.0 g of the dried and sieved particles was placed in a glassvessel and 10.0 g of acetonitrile were added. The mixture was placed ona laboratory shaker for 1 hour to extract leachable monomers. After 1hour, the particles were separated by filtration and the resulting clearliquid was directly injected into the HPLC. Measurement in regard tostandard solutions, containing defined amount of Ethoxylated Bisphenol Adimethacrylate (EBA) CAS: 41637-38-1 were conducted to determine theamount of extractable EBA. The amount of extractable monomers wascalculated from the amount of extractable EBA. The degree ofpolymerization of monomers represents the amount of non extractablemonomers in relation to the initial amount of monomers used forgranulation.

Method 2:

7.5 g of the dried and sieved particles were placed in a centrifugationtube and 30 mL of acetone were added. The mixture was put on alaboratory shaker for 20 minutes and then centrifuged for 30 minutes at5000 rpm. The clear supernatant was isolated by decantation. Theextraction procedure (adding acetone, shaking, centrifugation anddecantation) was repeated 2 times with 30 ml acetone each. The decantedsolutions were collected and combined, and the solvent removed bydistillation at 40° C. and 50 mbar. The remaining residue was analyzedgravimetrically and represents the amount of extractable monomers. Themeasurement was conducted twice. The degree of polymerization ofmonomers represents the amount of non extractable monomers in relationto the initial amount of monomers used for granulation.

Example 9—Scanning Electron Microscopy Pictures

Scanning electron microscopy (SEM) pictures were taken using a Ultrahigh resolution FESEM from Zeiss.

Example 10—Agglomeration/Granulation Using a Büchi Mini Spray Dryer(Büchi Minisprühtrockner B-290)

A certain amount of modified particles (strontium aluminum silicateglass or barium aluminum borosilicate glass—median particle sized_(50,3) and amount see table—modified with 3.1 wt.-%3-Methacryloylpropyltrimethoxysilane CAS: 2530-85-0) were mixed with adefined amount of acetone (see table 4) using a magnetic or mechanicalstirrer. A mixture of polymerizable methacrylate monomers (e.g. themonomer mixture of the commercially available material Dyract® eXtra,comprising urethane dimethacrylate, carboxylic acid modifieddimethacrylate, Triethyleneglycol dimethacrylate, Trimethacrylate resin,dimethacrylate resin, campherquinone, ethyl-4(dimethylamino)benzoate,butylated hydroxyl toluene, UV stabilizer, see table 4) were added tothis mixture together with a radical initiator WAKO V-601 (CAS:2589-57-3). The mixture was homogenized by stirring and then injectedinto the spray dryer using its internal peristaltic pump and a 2component jet nozzle from Büchi (nozzle diameter=1.4 mm) (processparameters see table 1). The aspirator efficiency was set to 100%. Theprocess parameters used are listed in table 4. For all experiments,nitrogen was used as the carrier gas (drying and atomization gas).

After the agglomerated particles were spray dried, the agglomeratedparticles were collected and placed in a three necked round bottomflask. The flask was purged with argon gas for at least 5 minutes andtightly sealed. The flask was put in an oven at 100° C. for 3.3 hours.The obtained particles were then sieved through a 180 μm sieve and themedian particle size (d_(50,3)) measured according to the proceduredescribed before. The median particle sizes (d_(50,3)) of theagglomerated particles are listed in table 4. The degree ofpolymerization of the monomers was determined using the above mentionedmethods 1 or 2. SEM pictures of the agglomerated fillers are shown inFIG. 4:

TABLE 4 List of variable amounts of modified particles, monomer mixture,variable process parameters, resulting median particle size (d₅₀) anddegree of polymerization of monomers Total Median Degree of amountparticle Span of polymer- Amount of Inlet size of particle izationmodified mono- Amount Amount Pump Nozzle Air flow temper- agglomeratedsize of glass d_(50,3)/ mers/ acetone/ Wako rate/ diameter/ (Rotameter)/ature/ Yield/ particles distri- monomers/ Experiment filler/g μm g gV-601/g % mm mm ° C. % (d₅₀)/μm bution % AZI-01-45-01 250 0.6 37.5³⁾140.2 0.43 60 1.4 50 120 91 12.67 ± 0.51  3.21 91¹⁾ AZI-01-46-01 350 0.635.0³⁾ 210.1 0.40 60 1.4 50 120 94 10.66 ± 0.13  3.60 88¹⁾ SNO-1-71-1350 0.6 70.0³⁾ 182.5 0.80 60 1.4 50 120 91 29.02 ± 8.93  3.41 92²⁾AZI-01-58-01 300 0.6 45.0³⁾ 168.3 0.51 50 0.7 50 120 91 9.79 ± 0.20 3.0789²⁾ AZI-01-59-01 300 0.6 45.0³⁾ 168.3 0.51 50 0.7 50 120 91 9.74 ± 0.483.62 89²⁾ SNO-1-87-01 150 0.5 22.5⁴⁾ 153.7 0.25 60 1.4 50 110 90 8.05 ±0.39 2.26 93¹⁾ AZI-01-136-01 300 0.5 44.8⁴⁾ 307.5 0.51 60 1.4 50 110 949.08 ± 0.24 4.21 Not determined ¹⁾Method 1 ²⁾Method 2 ³⁾mixture of thecommercially available material Dyract ® eXtra, comprising urethanedimethacrylate, carboxylic acid modified dimethacrylate,Triethyleneglycol dimethacrylate, Trimethacrylate resin, dimethacrylateresin, campherquinone, ethyl-4(dimethylamino)benzoate, butylatedhydroxyl toluene, UV stabilizer ⁴⁾mixture of ethoxylatedBisphenol-A-dimethacrylate, urethane dimethacrylate, Trimethacrylateresin, ethyl-4(dimethylamino)benzoate

Example 11—Paste Preparation Using Agglomerated Filler

Pastes were produced by placing a mixture of polymerizable methacrylatemonomers (e.g. the monomer mixture of the commercially availablematerial Dyract® eXtra, comprising urethane dimethacrylate, carboxylicacid modified dimethacrylate, Triethyleneglycol dimethacrylate,Trimethacrylate resin, dimethacrylate resin, campherquinone,ethyl-4(dimethylamino)benzoate, butylated hydroxyl toluene, UVstabilizer; or the monomer mixture of the commercially availablematerial Ceram.X, comprising methacrylate modified polysiloxane,dimethacrylate resins, fluorescent pigment, UV stabilizer, stabilizer,campherquinone, ethyl-4(dimethylamino)benzoate) in an IKA laboratorykneader and adding a certain amount of a mixture of granulated fillerand a further filler (see table 3) in portions under kneading to theresulting mixture. After the complete addition of the filler, furtherkneading steps were applied to ensure suitable distribution of thefiller within the final formulation.

TABLE 5 Formulation analysis Stickiness⁵⁾ Filler Consistency⁴⁾ Number ofFlexural Filler 1: granulated filler Filler 2¹⁾: content 23° C. 37° C.adhesive Shrinkage⁶⁾ strength⁷⁾ Experiment wt.-% wt.-% d₅₀ [μm] wt.-% mmmm breaks % MPa AZI-01-51-01 SNO-1-71-1 60  40³⁾ 0.6 76.8 31 36 0 2.4 90AZI-01-54-01 AZI-01-45-01 45  55³⁾ 0.6 76.8 25 32 0 2.5 102 AZI-01-49-01AZI-01-46-01 60  40³⁾ 0.6 76.8 14 21 6 2.6 91 KJ-19-123-1 SNO-1-71-1 30 70³⁾ 0.6 76.8 25 31 0 2.5 108 KJ-19-121-1 AZI-01-46-01 30  70³⁾ 0.676.8 21 26 0 2.6 126 KJ-19-141-1 — 0 100³⁾ 0.6 76.1 20 27 0 2.7 117KJ-19-137-1 AZI-01-58-01 50  50³⁾ 0.6 78.4 20 28 6 2.3 105AZI-01-59-01²⁾ AZI-01-68-01 AZI-01-58-01 40  60³⁾ 0.6 77.8 22 29 5 2.4111 AZI-01-59-01²⁾ AZI-01-69-01 AZI-01-58-01 60  40³⁾ 0.6 77.7 21 37 62.3 108 AZI-01-59-01²⁾ KJ-19-183-01 SNO-1-87-01 50 50 0.5 73.8 18 21 62.0 94 ¹⁾same filler used to generate granulated filler table 1 ²⁾a 1:1mixture of AZI-01-58-01 and AZI-01-59-01 was used ³⁾Filler 1 and Filler2 were mixed prior to addition to formulation using a Willy A. BachofenTurbula T2F at 50 rpm for 10 minutes ⁴⁾Determined by placing a weight ofapprox. 575 g on a specimen with a volume of 0.5 mL (∅ 7.0 mm) for 120seconds at 23° C. or 37° C., and measure diameter of the resulting roundround disk in mm. ⁵⁾Determined using a by placing the material betweentwo metal plates (one fixed lower plate, one movable upper plate). Thedistance between both plates is set to 2 mm. After annealing thematerial to 35° C. for 5 minutes, the upper plate is retracted at aconstant speed (approx.. 1 mm/s) and the breakage of the material isobserved. A break of the material, during retraction of the upper plate)within the material itself constitutes a cohesive break, while abreakage of the material from either plate constitutes an adhesivebreak. A total of 6 measurements was made. A high number of adhesivebreaks (maximum 6 out of 6) shows a low stickiness of the material.⁶⁾Determined by measuring the change in density before and afterpolymerization and calculating the volumetric shrinkage using theArchimedes hydrostatic uplift principle ⁷⁾Determined according to ISO4049:2009

Comparative Example 1—U.S. Pat. No. 4,781,940 “Method for ProducingFiller for Microfilled Dental Composite Material”

According to Example 1 of U.S. Pat. No. 4,781,940 disclosing thepreparation of a filler for a dental composite material, one hundredgrams of OX-50 silica that had been treated with 5%, by weight, of A-174silane is placed in a mixing vessel with a solution containing 120 gramsof methylene chloride and 25 grams of the following monomer mixturecontaining 61.8 wt.-% bis-GMA, 6.9 wt.-% bisphenol-A dimethacrylate,29.4 wt.-% triethylene glycol dimethacrylate and 2.0 wt.-% radicalinitiator.

The slurry is poured into a tray and the methylene chloride isevaporated by allowing the tray and its contents to stand in air atambient temperature (about 23° C.) for 16 hours.

After evaporating the methylene chloride solvent, the coated silica isthen passed through a 165 mesh screen. The sieved material is heated to120° C. for 4 hours in a vacuum oven under a vacuum of 30 mm Hg(absolute pressure).

After the heating step, the powder is cooled, sieved again through a 100μm sieve. The sieved material was heated to 120° C. for 4 hours in avacuum oven, or alternatively in an inert atmosphere. After heating, thematerial was sieved again through a 100 μm sieve for providing thefiller for a dental composite material.

SEM images of the materials were taken to illustrate the morphology ofthe particles. Scanning electron microscopy (SEM) pictures were takenusing a Ultra high resolution FESEM from Zeiss. The results are shown inFIG. 5. A fluid-like movement of the particles cannot be observed.

Example 12—Filler Preparation According to the Present Invention UsingOX-50

SNO-1-92-1: 40 g of treated OX-50 (treated with 5 wt.-% A-174silane=gamma-methacryloxypropyltrimethoxysilane) were mixed with 48 gmethylene chloride and 10 g of a mixture containing 61.8 wt.-% bis-GMA,6.9 wt.-% bisphenol-A dimethacrylate, 29.4 wt.-% triethylene glycoldimethacrylate and 2.0 wt.-% radical initiator.

The mixture was homogenized by stirring and then injected into the spraydryer using its internal peristaltic pump and a 2 component jet nozzlefrom Büchi (nozzle diameter=1.4 mm), setting the pump rate to 20%, therotameter to 55 mm and the inlet temperature to 75° C. The aspiratorefficiency was set to 100%. Nitrogen was used as the carrier gas (dryingand atomization gas).

After the agglomerated particles were spray dried, the agglomeratedparticles were collected and placed in a three necked round bottomflask. The flask was purged with argon gas for at least 5 minutes andtightly sealed. The flask was put in an oven at 100° C. for 3.3 hours.

SEM images of the materials were taken to illustrate the morphology ofthe particles. The results are shown in FIG. 6.

Comparative Example 2—Filler Preparation According to U.S. Pat. No.4,781,940 Using Modified Dental Glass Particles

SNO-1-74-1: 150 g of a milled and silanated dental glass (strontiumaluminum silicate glass—median particle size d3.50=0.6 μm were mixedwith 78.6 g acetone and 22.5 g of a mixture of polymerizablemethacrylate monomers (monomer mixture of the commercially availablematerial Dyract® eXtra, comprising urethane dimethacrylate, carboxylicacid modified dimethacrylate, Triethyleneglycol dimethacrylate,Trimethacrylate resin, dimethacrlyate resin, campherquinone,ethyl-4(dimethylamino)benzoate, butylated hydroxyl toluene, UVstabilizer) and 0.26 g radical initiator.

The slurry was poured into a tray and acetone was evaporated by lettingthe tray stand in air at ambient temperatures (approx. 23° C.) for 16hours. After evaporating the acetone solvent, the coated glass particleswere passed through a 100 μm sieve. The sieved material was heated to120° C. for 4 hours in a vacuum oven, or alternatively in an inertatmosphere.

SEM images of the materials were taken to illustrate the morphology ofthe particles. The results are shown in FIG. 7. A fluid-like movement ofthe particles cannot be observed.

Example 13—Filler Preparation According to the Present Invention UsingModified Dental Glass Particles

AZI-01-45-01: 250 g of a milled and silanated dental glass (strontiumaluminum silicate glass—median particle size d3.50=0.6 μm were mixedwith 140 g acetone and 37.5 g of a mixture of polymerizable methacrylatemonomers (monomer mixture of the commercially available material Dyract®eXtra, comprising urethane dimethacrylate, carboxylic acid modifieddimethacrylate, Triethyleneglycol dimethacrylate, Trimethacrylate resin,dimethacrlyate resin, campherquinone, ethyl-4(dimethylamino)benzoate,butylated hydroxyl toluene, UV stabilizer) and 0.43 g radical initiator.

The mixture was homogenized by stirring and then injected into the spraydryer using its internal peristaltic pump and a 2 component jet nozzlefrom Büchi (nozzle diameter=1.4 mm), setting the pump rate to 60%, therotameter to 50 mm and the inlet temperature to 120° C. The aspiratorefficiency was set to 100%. Nitrogen was used as the carrier gas (dryingand atomization gas).

After the agglomerated particles were spray dried, the agglomeratedparticles were collected and placed in a three necked round bottomflask. The flask was purged with argon gas for at least 5 minutes andtightly sealed. The flask was put in an oven at 100° C. for 3.3 hours.

SEM images of the materials were taken to illustrate the morphology ofthe particles. The results are shown in FIG. 8.

Based on the SEM pictures shown in FIGS. 5 to 8, a clear difference inthe morphology of the prepared agglomerated fillers can be seen. Whereasin case of U.S. Pat. No. 4,781,940 no defined shape of the agglomeratedparticles can be seen, the method according to the present inventionclearly shows the formation of spherical particles.

The invention claimed is:
 1. A process for preparation of compositefiller particles, comprising the steps of: (a) coating a particulateglass filler having a median particle size (D50) of from 100 to 1200 nmwith a coating composition containing a film-forming agent forming acoating layer on the surface of the particulate glass filler, saidcoating layer displaying reactive groups on the surface of the coatinglayer, said reactive groups being selected from addition polymerizablegroups and step-growth polymerizable groups, thereby forming a coatedparticulate glass filler; subsequently or concurrently (b) agglomeratingthe coated particulate glass filler, for providing a granulation of thecoated particulate glass filler wherein the granulation contains thecoated particulate glass filler particles separated from and connectedto each other by at least one coating layer, whereby the at least onecoating layer may be crosslinked by crosslinking groups obtained byreacting the reactive groups of the coated layer, whereby theagglomeration is carried out by spray agglomeration; wherein theprepared composite filler particles have a median particle size (D50) offrom 1 to 70 μm; wherein reactive groups are transformed intocrosslinking groups by addition polymerization or step-growthpolymerisation; and wherein the particulate glass filler is the maincomponent by volume of the composite filler particles.
 2. The processaccording to claim 1, wherein said reactive groups are selected fromstep-growth polymerizable groups.
 3. The process according to claim 1,wherein the coating composition contains a film-forming agent forming acovalent bond with the particulate glass filler.
 4. The processaccording to claim 3, wherein the covalent bond is obtained by thereaction of a hydroxyl group on the surface of the particulate glassfiller and a silane group.
 5. The process according to claim 1, whereinthe maximum thickness of the coating layer on the particulate glassfiller is less than the median particle diameter (D50) of theparticulate glass filler.
 6. The process according to claim 1, whereinthe coating composition comprises one or more polymerizable monomers, apolymerization initiator, and a solvent.
 7. The process according toclaim 1, wherein the coating composition has a dynamic viscosity in therange of from 0.0001 to 15 Pas (23° C.).
 8. The process according toclaim 1, wherein the process is carried out in a fluidized bed.
 9. Theprocess according to claim 1, wherein the particulate glass fillerparticles has a median particle size (D50) in the range of from 2 μm to20 μm.
 10. The process according to claim 1, which further includes astep of treating the composite filler particles with a silanating agentor a surface active agent.
 11. The process according to claim 1, whereinagglomerating the coated particulate glass filler occurs in the presenceof a further crosslinking agent.
 12. The process according to claim 1,wherein agglomerating the coated particulate glass filler occurs in thepresence of a further particulate glass filler not displaying reactivegroups.
 13. The process according to claim 11, wherein the at least onecoating layer is crosslinked by crosslinking groups obtained by reactingthe reactive groups and the further crosslinking agent.
 14. The processaccording to claim 1, further comprising at least one of milling,classifying or sieving the granulation of the coated particulate glassfiller.
 15. The process according to claim 1, further comprisingcrosslinking the granulation of the coated particulate glass filler. 16.The process according to claim 1, wherein the crosslinking of the atleast one coating layer can occur during agglomeration or afteragglomeration.